Currently, in the applications as the constituent materials for absorbing articles such as paper diapers, sanitary napkins and incontinence pads, water-absorbing agents containing water-absorbing resins as a main component are widely used, in addition to hydrophilic fibers such as pulp.
Known examples of water absorption characteristics that are desired in the water-absorbing resins include centrifuge retention capacity, absorption against pressure, liquid permeability, rate of water absorption, water-extractable, and gel strength, and many suggestions have been made so far on the methods for measuring or enhancing these physical properties (methods for producing water-absorbing resins), as well as patents for parameters that define these physical properties (water-absorbing resins or water-absorbing agents).
Among the physical properties described above, the centrifuge retention capacity is the most fundamental property, and the methods for measuring the property are roughly classified into methods in which a water-absorbing resin is caused to swell in physiological saline and then dehydration is carried out by centrifugation (CRC method: Centrifuge Retention Capacity method; centrifugation method), and methods in which dehydration is carried out by suspending the water-absorbing resin, without performing centrifugation (FSC method: Free Swell Capacity method; also known as TB method). The absorption capacities obtained by these measurement methods are generally known to correlate with each other (for example, FSC=1.02×CRC+11.32; Non-Patent Literature 1, page 152). However, since the absorption capacity according to the CRC method expresses the absorption capacity of a water-absorbing resin itself while sufficiently excluding the interstitial water at the time of swelling, evaluations according to the CRC method are generally used for the present, and the CRC method is employed in many Patent Literatures and in the step management for water-absorbing resins.
Furthermore, examples of the basic physical properties of a water-absorbing resin other than the centrifuge retention capacity (CRC) include the absorption against pressure (AAP) and liquid permeability (for example, SFC and GBP), and many suggestions have been made on a water-absorbing resin which has these physical properties controlled, or methods for controlling the physical properties.
For example, Patent Literature 1 discloses a method for producing a high liquid-permeable water-absorbing agent, in which when a water-absorbing agent is produced by mixing water-absorbing resin particles with a metal compound (a metal salt, a metal cation, or the like), the metal component is prevented from penetrating into the interior of the water-absorbing resin particles, and also the metal component uniformly adheres in the form of fine dots to the overall surface of the water-absorbing resin.
Patent Literature 2 discloses a method for producing a water-absorbing agent having the gel blocking properties, liquid permeability and liquid diffusibility improved all at the same time.
Patent Literature 3 discloses, as a method for producing a water-absorbing agent which exhibits high gel bed permeability (GBP), does not have a tendency of aggregation or caking, and does not have a tendency to form dust, a method for producing a superabsorbent which is re-wetted and surface-crosslinked, the method including a step for bringing a superabsorbent base polymer, an organic crosslinking agent and a polyvalent metal salt solution into contact in the presence of an alcohol, subjecting the mixture to a heat treatment to produce a dry superabsorbent that has been surface-crosslinked, and re-wetting the surface-crosslinked dry superabsorbent.
Patent Literature 4 discloses a method for producing a water-absorbing polymer structure having high absorption capacity, the method including steps of preparing an untreated water-absorbing polymer structure (Pu) having a particular retentivity; and bringing the untreated water-absorbing polymer structure (Pu) into contact with a permeability enhancing agent which is preferably a SiO compound, a salt containing a polyvalent (preferably trivalent) cation, or a mixture of a SiO compound and a salt containing a polyvalent (preferably trivalent) cation, in order to provide a water-absorbing polymer structure which is capable of enhancing the absorption and dispersion of a liquid when used in an absorptive structures such as, for example, a disposable diaper, as compared with conventional absorbing agents.
Patent Literature 5 discloses a method of obtaining a highly absorptive polymer having excellent liquid permeability, by mixing a polyvalent metal salt and a highly absorptive polymer by dry blending.
Furthermore, as methods for evaluating liquid permeability, many methods or technologies for improving liquid permeability under load or liquid permeability under no load, such as a SFC method (Saline Flow conductivity; Patent Literature 6) and a GBP method (Gel Bed Permeability; Patent Literatures 7 to 9). Furthermore, in regard to such physical properties, many combinations of plural parameters including liquid permeability have also been suggested, and there are known a technology for defining impact resistance (FI) (Patent Literature 10), a technology for defining the rate of water absorption (FSR and/or Vortex) (Patent Literature 11), and a technology for defining the product of liquid diffusion performance (SFC) and the core absorption amount after 60 minutes (DA60) (Patent Literature 12).